This invention relates to microwave PIN diode switches, and more particularly to a technique for providing impedance matching in such switches.
PIN diodes consist of heavily doped p.sup.+ and m.sup.+ end regions separated by a lightly doped region which can usually be regarded as intrinsic. If this center region is thick, e.g., 10-100 microms, the device is useful as a high-voltage rectifier with a low forward drop at high currents because of the conductivity modulation of the i region by the large number of carriers injected from the end regions. It is also known to use the PIN diode as a variable resistance at microwave frequencies. Because of the relatively long recovery time of the i layer, microwave frequencies will be too high for rectification to occur. At zero or reverse bias the intrinsic layer will represent a high resistance and, under forward bias, the injection and storage of carriers reduces the resistance of the intrinsic region to a very low level. These diodes can be used as microwave switches when driven with abrupt bias changes.
FIG. 1a shows a PIN diode having an anode 10 and a cathode 12. FIG. 1b shows the equivalent circuit of the PIN diode of FIG. 1a. The RF resistance R.sub.D of the PIN diode is a nonlinear resistance which varies as a function of the applied bias. The effective resistance of R.sub.D can vary from 0.5 ohms at full forward bias to 10 k ohms at zero bias.
Shown in FIG. 2 is a schematic diagram of a conventional reflective type microwave PIN diode switch. In order to permit the passage of RF power, the PIN diodes 14 and 16 should be reverse biased so that they represent open circuits. This is accomplished by supplying a negative potential at the DC bias input terminal. The DC bias is isolated from the RF signal by the .lambda./4 transmission line 18 which represents a short circuit for the DC bias current and an open circuit for the RF signal. The equivalent circuit for the switch of FIG. 2 when the PIN diodes are reverse biased is shown in FIG. 3a. A signal propagating along a transmission line having a characteristic impedance Z.sub.0 will encounter a .lambda./4 transmission line 20 having an impedance Z.sub.0 and the .lambda./4 transmission line 18 having an impedance Z.sub.1 with Z.sub.1 &gt;&gt; than Z.sub.0. Due to the its reflective RF termination through capacitor 19, the transmission line 18 will represent an open circuit to the RF signal and, consequently, the RF signal will effectively see only the .lambda./4 transmission line 20 with impedance Z.sub.0. The RF signal will propagate through the transmission line 20 and appear at the output of the switch. Since the input and output impedances of the switch are substantially Z.sub.0, impedance matching is achieved at both the input and output ports of the switch and the RF signal will propagate through the switch with little or no reflection.
When the conventional switch in FIG. 2 is used to block the RF signal, the diodes 14 and 16 are forward biased to represent short circuits on either side of the switch. This is accomplished by providing a positive DC bias signal through the .lambda./4 transmission line 18. The equivalent circuit of the switch of FIG. 2 and its RF "off" condition is shown in FIG. 3b. The diodes 14 and 16 are forward biased to represent approximately 0.5 ohms across the transmission path and the RF power on either side of the switch will be reflected by the severe impedance mismatch.
In some applications, this reflected RF energy could have a significant adverse effect on the surrounding circuitry. For example, in an 8.times.8 Microwave Switch Matrix (MSM), eight different RF input signals are simultaneously supplied to respective 8-way power dividers so that each RF input port is coupled to a respective row in an 8.times.8 array of power divider ports. At the output of the MSM are eight RF ports each connected to a respective 8-way power divider in a similar fashion as the input ports, and the output power dividers are arranged orthogonally to the input dividers. The 64 power divider ports are coupled through an 8.times.8 array of 64 PIN diode switches, so that any one of the eight RF input ports can be connected to any one of the eight RF outputs.
If conventional microwave switches such as shown in FIG. 2 are used in such a MSM, the "off" switches will reflect the RF energy back through the power dividers and the reflected RF energy will interfere with the RF signal at the corresponding input port. It has been discovered that this reflective energy will produce large insertion loss variations with frequency and an insertion loss scattering from path-to-path which are difficult to control over a wide frequency band. These insertion loss variations with frequency distort the communication signals of high capacity Time Division Multiple Access (TDMA) carriers and produce intersymbol interference. Further, in Satellite Switched Time Division Multiple Access (SS-TDMA) operation, the insertion loss scattering from path-to-path would require adaptive gain control at the eight outputs of the MSM in order to insure a constant transmitted energy within a SS-TDMA frame.